1. Field of the Invention
The invention relates to a process for atomizing liquid feed to refinery process, such as distillation columns or an FCC riser reactor and to an apparatus for atomizing liquids.
2. Description of Related Art
Refiners have long known that feed atomization in the base of FCC riser reactors is a problem. It is difficult to contact many tons per hour of hot, regenerated cracking catalyst with large volumes of heavy oil feed, and ensure complete vaporization of the feed in the base of the riser reactor. Part of the problem is use of heavier feeds in FCC units. Many PCC's now process significant amounts, on the order of 5-20% of resid or non-distillable material. These materials are almost impossible to vaporize in fractionators, so vaporizing them in less than a second or so in an FCC riser reactor is a formidable task.
Part of the problem is that feed nozzles which were perfectly satisfactory for adding a readily vaporizable feed, such as a gas oil, are no longer adequate for heavier feeds. The problems are twofold: the heavier feeds are harder to vaporize because of their high boiling points, and the heavy feeds are harder to atomize because of their high viscosity even at the high temperatures existing in FCC riser reactors.
Efforts of refiners to cope with heavier feeds, or improve the vaporization of lighter ones, will be briefly reviewed.
Some of the efforts at improving regenerated catalyst/feed contacting were on the catalyst side, i.e., the use of lift gas to smoothly lift catalyst up into the riser. Other approaches assumed that catalyst distribution will be poor and forced oil distribution (via multiple nozzles) to be equally poor.
Increased steam addition is common practice for dealing with heavier feeds. Increased atomization steam usually leads to increased pressure drop across the existing feed nozzles, and increased sour water production. Although some improvement in feed dispersion is usually achieved, the problems of increased sour water production, and limits on pressure at which feed can be delivered to the nozzle inlets, limit the improvement from merely increasing steam rate.
In addition to focussing on catalyst/oil distribution and making existing nozzles work better with more steam refiners considered other nozzle designs. Some very effective nozzle designs were developed but required unusually high pressure drops across the nozzle for effective operation. Refiners like to operate the feed nozzles at as low a pressure as possible, both to save on capital equipment and operating cost. Many units are limited in the feed pressure, and major capital improvements are needed to use high pressure drop nozzles. Development of high pressure liquid streams also consumes a considerable amount of energy.
A good overview of developments in nozzles is presented in Fluid Catalytic Cracking Report: 50 Years of Catalytic Cracking; A. A. Avidan et al, Oil & Gas Journal, Jan. 8, 1990, at page 50. Open pipe or slotted, impact, spiral and critical venturi nozzles have all been tried.
The open pipe or slotted nozzle gives coarse irregular droplet sizes and is not ideally suitable for injecting heavy feeds into an FCC riser reactor.
Critical venturi nozzles, where an oil and steam mixture is passed through a critically sized venturi section into a larger, intermediate chamber and are discharged through a restricted nozzle can achieve very small droplet sizes. These droplets can be completely vaporized in less time than the droplets produced by spiral nozzles, but such nozzles require high liquid pressure drops, and develop a narrow spray pattern.
A hybrid approach, use of high velocity steam (1000 to 1800 ft/sec) to atomize a low velocity oil stream (20 to 50 ft/sec) was disclosed in U.S. Pat. No. 3,654,140, which is incorporated herein by reference. The high velocity steam imparts energy to the low velocity liquid. FIG. 2 of '140 shows oil discharged as a cone of liquid which is broken into droplets by a high velocity steam stream enveloping the cone. The design was an improvement over the nozzle shown in U.S. Pat. No. 3,152,065, an earlier nozzle developed by the same assignee, wherein liquid passed through an annular region about a central steam pipe to contact an expanding steam stream upstream of a restricted opening. Imparting a centrifugal component to the liquid stream probably threw the liquid to the sides of the nozzle, and may have impaired atomization. The liquid went out as a cone and was not impinged by the high velocity steam stream in the central region of the nozzle.
Although there are myriad nozzle designs, many of which are unique and hard to classify, they can be more or less arbitrarily classified as relying on one or more of the following mechanisms for drop formation.
Expansion is the most widely used form of FCC feed nozzle. A mixture of 1-5 wt. % atomizing steam and the heavy, preheated feed, pass through a slot or circular orifice to form a spray.
Mixing/Expansion involves use of swirl vanes followed by an orifice.
Shear/Breaking atomizes liquid by passing it through a spiral-shaped orifice. Spiral FCC feed nozzles are examples.
Impingement nozzles pass an atomizing gas stream through multiple orifices to strike a liquid stream. The Lechler nozzle is a good example of this type of nozzle.
Breaking/Mixing nozzles atomize by the high velocity impact of two phases. The Snowjet nozzle is of this type.
Although it might seem possible to simply stack these types of nozzles in series, and thereby improve atomization, this is not possible in practice. Additional stages may or may not improve atomization, but will always increase pressure drop and this alone will usually prevent simple stacking of these unit operations. Many attempts to improve nozzle performance, as by stacking atomizing devices, degrade performance. We tried adding swirl vanes to a nozzle, and found that they actually made the distribution worse.
In FCC units, the nozzles must also be robust and reliable, as run lengths of one or two years or more are common. FCC units have additional constraints. The hydrocarbon feeds are supplied at a certain pressure, usually around 50-200 psig. Because of the large size of these streams, and the cost of energy needed to pump the feed to higher pressures, and site constraints which frequently prevent easy addition of high pressure pumps, it is very important to have a nozzle which will work well with relatively low oil pressures.
High pressure steam is usually readily available, and is a preferred atomization medium, but refiners usually want to minimize its use. High pressure steam is a valuable commodity in a refinery, and its use fills much of the FCC riser and downstream processing equipment with an inert material. Refiners are also reluctant to use too much steam, or to have too high an exit velocity from the nozzle, because of catalyst erosion, and riser impingement concerns previously noted.
Nozzle exit velocities, regardless of the internal oil or steam pressure required by the nozzle, should not be excessive because high velocity streams can shorten catalyst life, by attrition.
An additional constraint is that the material exiting the nozzle should contact as much of the catalyst flowing by the nozzle as possible, without carrying through the catalyst to a side portion of the riser.
It is also beneficial if the nozzles used, whether vertical or side mounted, are relatively small, so that flow of hot catalyst up the riser is not disrupted.
We realized a different approach was needed in nozzle design. Our nozzle combines several different mechanisms for droplet formation, and achieves efficient atomization, with relatively low amounts of atomizing fluid, without requiring inordinately high oil feed pressures, or impinging on the wall of the riser reactor. Our nozzle design does not exhibit slugging characteristics, and can be mounted either vertically or at a slant, which permits its use in riser reactors having side mounted nozzles. The design is both robust and compact, and catalyst flows readily around it.